The invention relates to a pressure limiting valve with control and valve pistons which move separately against a valve spring or a regulating spring, whereby the valve piston responds by exposing the discharge openings, after the control piston with its blind hole, which is larger than the through-bore, has responded first and has thus enabled the valve piston with its significantly larger diameter to open against the pressure of the regulating spring and of the pressure medium present in the compensation chamber.
Pressure limiting valves are used in particular in underground mining for protecting the hydraulic support from sudden overload, i.e. particularly rock bursts. German Offenlegungsschrift 28 30 891 discloses a pressure limiting valve wherein overpressures in the hydraulic system are reduced by way of a valve spring arranged between screw plug and valve piston. To the valve plate or spring plate is molded a conical or spherical closure element which is lifted from the valve seat if an overload occurs. Onto the piston is molded a damping cylinder which limits the flow through-opening. The closing safety of these known pressure limiting valves is insufficient. In addition, a correct design of the spring is very difficult, although it is the precondition for a safe response of pressure limiting valves. German Offenlegungschrift 33 14 837, German Offenlegungsschrift 39 22 894, and German Offenlegungsschrift 39 22 094 disclose pressure limiting valves permitting a flow volume of approximately 1,000 liters per minute, whereby these valves have distinctly small dimensions. This is achieved in that the valve piston is constructed as a pipe sleeve which works together with a control piston in such a way that both can be moved together as a structural unit inside the internal bore of the valve case. The known control piston has a blind hole and radial bores through which the pressure medium is able to flow into the spring chamber when the valve responds. The necessary differential surfaces are realized by way of the stepped control piston or the different diameters of control piston and valve piston. The disadvantage in these pressure limiting valves consists of the closing values which are still inadequate, whereby in particular a reclosing of the valve occurs and thus results in a fluttering of the valve piston, since both pistons must move together. The valve spring which is used must be relatively large and long, thus necessitating a more expensive construction for such pressure limiting valves. But the disadvantage in all pressure limiting valves is primarily that, due to the predefined construction, they have a specific output which cannot be changed as such.